We report on a BeppoSAX Concentrator Spectrometer observation of the
super-soft source (SSS) CAL87. The X-ray emission in SSS is believed to arise
from nuclear burning of accreted material on the surface of a white dwarf (WD).
An absorbed blackbody spectral model gives a chi^2_v of 1.18 and a temperature
of 42 +/- ^13 _11 eV. However, the derived luminosity and radius are greater
than the Eddington limit and radius of a WD. Including an O viii edge at 0.871
keV gives a significantly better fit (at > 95% confidence) and results in more
realistic values of the source luminosity and radius. We also fit WD atmosphere
models to the CAL87 spectrum. These also give reasonable bolometric
luminosities and radii in the ranges 2.7-4.8 10^{36} erg/s and 8-20 10^7 cm,
respectively. These results support the view that the X-ray emission from CAL87
results from nuclear burning in the atmosphere of a WD.Comment: 4 pages. Accepted for publication in A&A (Letters

Bavdaz, M.

Hartmann, H. W.

Heise, J.

Kahabka, P.

Martin, D. D. E.

Mineo, T.

Parmar, A. N.

English

arXiv

Parmar, A.

Hartmann, H.W.

Martin, D.D.E.

International Migration, Integration and Social Cohesion online publications

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)A BeppoSax observation of the supersoft source CAL87Parmar, A.; Kahabka, P.; Hartmann, H.W.; Heise, J.; Martin, D.D.E.; Bavdaz, M.; Mineo, T.Publication date1997Published inAstronomy & AstrophysicsLink to publicationCitation for published version (APA):Parmar, A., Kahabka, P., Hartmann, H. W., Heise, J., Martin, D. D. E., Bavdaz, M., & Mineo,T. (1997). A BeppoSax observation of the supersoft source CAL87. Astronomy &Astrophysics, 323, L33-L36.General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:25 Jul 2022Astron. Astrophys. 323, L33–L36 (1997) ASTRONOMYANDASTROPHYSICSLetter to the EditorA BeppoSAX observation of the super-soft source CAL 87A.N. Parmar1, P. Kahabka2, H.W. Hartmann3, J. Heise3, D.D.E. Martin1, M. Bavdaz1, and T. Mineo41 Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands2 Astronomical Institute and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403,1098 SJ Amsterdam, The Netherlands3 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands4 IFCAI/CNR, via U. Malfa 163, I-90146 Palermo, ItalyReceived 15 April 1997 / Accepted 6 May 1997Abstract. We report on a BeppoSAX Concentrator Spectrom-eter observation of the super-soft source (SSS) CAL 87. TheX-ray emission in SSS is believed to arise from nuclear burn-ing of accreted material on the surface of a white dwarf (WD).An absorbed blackbody spectral model gives a χ2ν of 1.18 anda temperature of 42±1311 eV. However, the derived luminosityand radius are greater than the Eddington limit and radius ofa WD. Including an O viii edge at 0.871 keV gives a signifi-cantly better fit (at >95% confidence) and results in more re-alistic values of the source luminosity and radius. We also fitWD atmosphere models to the CAL 87 spectrum. These alsogive reasonable bolometric luminosities and radii in the ranges2.7–4.8×1036 erg s−1 and 8–20×107 cm, respectively. Theseresults support the view that the X-ray emission from CAL 87results from nuclear burning in the atmosphere of a WD.Key words: X-rays: stars − accretion − binaries:close −stars:individual (CAL 87) − white dwarfs1. IntroductionThe Einstein observatory performed a survey of the Large Mag-ellanic Cloud (LMC) in which two sources with unusually softspectra, CAL 83 and CAL 87 were detected (Long et al. 1981).These sources emit little or no radiation at energies ∼>1 keVand became known as “super-soft” sources (SSS). SubsequentROSAT observations have revealed approximately 30 similarsources located in the Galaxy, the Magellanic Clouds, a globularcluster and M31 (see Kahabka 1995; Kahabka & Trümper 1996for recent reviews). SSS are hard to detect in the galactic planeSend offprint requests to: A.N. Parmar: aparmar@astro.estec .esa.nldue to the effects of interstellar absorption. Absorbed black-body spectral models give typical temperatures of ∼40 eV andbolometric luminosities of ∼1038 erg s−1.SSS were originally interpreted as due to scattering from anaccretion disk corona (e.g., Smale et al. 1988), or accreting neu-tron stars radiating near or above the Eddington limit (Greineret al. 1991; Kylafis & Xilouris 1993). Van den Heuvel et al.(1992) proposed that these are systems undergoing steady nu-clear burning of hydrogen accreted onto the surface of a whitedwarf (WD) with masses in the range 0.7–1.2 M. The masstransfer from a main-sequence or sub-giant companions is un-stable on a thermal time scale and for a narrow range of accretionrates, steady nuclear burning can take place. Evolutionary sce-narios for such systems are discussed in Rappaport et al. (1994).It is unlikely that SSS compose a homogeneous class and oneway of probing the nature of individual sources is by searchingfor the characteristic spectral signatures of nuclear burning on aWD. This burning takes place deep within the WD atmosphereat a large energy dependent optical depth. Photoelectric absorp-tion by highly ionized metals in the atmosphere can produceedges in the X-ray spectrum. These effects have been modeledassuming Local Thermodynamic Equilibrium (LTE) by Heiseet al. (1994) and more recently extended to the non-LTE (NLTE)case by Hartmann & Heise (1997).CAL 87 exhibits both X-ray and optical eclipses with a pe-riod of 10.6 hrs (Callanan et al. 1989; Cowley et al. 1990;Schmidkte et al. 1993; Kahabka et al. 1994), indicating an orbitalinclination of>70◦. The optical lightcurve shows a deep asym-metric primary minimum with a shallow, variable, secondaryminimum while the X-ray eclipse is narrower and shallow. Theoptical modulation may be due to obscuration by a structured ac-cretion disk (Schandl et al. 1997). Fitting an absorbed blackbodymodel to a 37 ksec exposure ROSAT Position Sensitive Pro-portional Counter (PSPC; 0.1–2.5 keV; Trümper 1983) CAL 87spectrum gives a best-fit temperature, T, of 35 eV and an equiv-LETTERL34 A.N. Parmar et al.: A BeppoSAX observation of the super-soft source CAL 87Fig. 1. Absorbed blackbody (left panel) and blackbody with a 0.871 keV absorption edge (right panel) model fits to the LECS CAL 87 spectrumFig. 2. Absorbed LTE (left panel) and NLTE (right panel) model fits to the LECS CAL 87 spectrumalent hydrogen column density, NH, of 1× 1022 H atoms cm−2(Hartmann & Heise 1997). CAL 87 has been a persistent X-raysource since its discovery in 1980.2. ObservationsThe scientific payload of the BeppoSAX X-ray Astronomy Satel-lite (Boella et al. 1997a) comprises four coaligned Narrow FieldInstruments, or NFI, including the Low Energy and MediumEnergy Concentrator Spectrometers (LECS and MECS). TheLECS is an imaging gas scintillation proportional counter sen-sitive in the energy range 0.1–10.0 keV with a circular fieldof view of 37′ diameter (Parmar et al. 1997). The backgroundcounting rate is 9.7 × 10−5 arcmin−2 s−1 in the energy range0.1–10.0 keV. The LECS energy resolution is a factor∼2.4 bet-ter than that of the ROSAT PSPC, while the effective area isbetween a factor ∼6 and 2 lower at 0.28 and 1.5 keV, respec-tively. CAL 87 was observed by the LECS as part of the ScienceVerification Phase between 1996 October 27 02:14 to October28 23:42 UTC. Due to the failure of a ground segment linkand an instrument anomaly, data between October 28 04:54 and10:55 UTC were lost. Good data were selected from intervalswhen the minimum elevation angle above the Earth’s limb was>4◦ and when the instrument’s settings were nominal usingthe SAXLEDAS 1.4.0 data analysis package. Since the LECSwas only operated during satellite night-time, this gave a totalon-source exposure of 39 ksec. The MECS is sensitive in theenergy range 1.5–10 keV, with energy and angular resolutionssimilar to the LECS (Boella et al. 1997b). The MECS observedLETTERA.N. Parmar et al.: A BeppoSAX observation of the super-soft source CAL 87 L35CAL 87 for a total exposure of 120 ksec, but did not detect thesource.Examination of the LECS image shows a source at a posi-tion consistent with (32′′ distant) that of CAL 87. A spectrumwas extracted centered on the source centroid using a radius of8′. This radius was chosen to include 95% of the 0.28 keV pho-tons. The spectrum was rebinned to have >20 counts in eachbin to allow the use of the χ2 statistic. The XSPEC 9.01 pack-age (Arnaud 1996) was used for spectral analysis together withthe response matrix from the 1996 December 31 release. Back-ground subtraction was performed using a standard blank fieldwith a 46 ksec exposure. The CAL 87 count rate above back-ground was 0.0076±0.0011 s−1. Examination of the extractedspectrum shows that the source was only detected in a narrowenergy range (see Figs. 1 and 2) and only the 17 rebinned chan-nels corresponding to energies between 0.2 and 1.5 keV wereused for spectral fitting.2.1. Spectral fitsIn order to compare the LECS spectrum with those obtainedfrom previous observations, we first fit an absorbed blackbodyspectral model to the data (Fig. 1). The photoelectric absorptioncoefficients of Morisson & McCammon (1983) together withthe solar abundances of Anders & Grevesse (1989) were used.An acceptable fit is obtained with χ2ν of 1.18 for 14 degrees offreedom (dof). The best-fit parameters are given in Table 1. Adistance of 50 kpc is assumed in order to derive the WD radius,R, and luminosity, L, and all uncertainties are quoted at 68%confidence. The spectrum shows evidence for an abrupt cutoff∼>0.8 keV and so an O viii edge with absorption depth τ at afixed energy of 0.871 keV, was added to the model. This edge isthe dominant spectral feature at energies∼>0.8 keV in many WDmodel atmosphere calculations (e.g. Heise et al. 1994; White etal. 1995). Including the edge gives a higher best-fit temperatureand improves the fit quality, resulting in a χ2ν of 0.93 (Table 1).The value of the F statistic of 4.76 indicates that the addition ofthe edge is significant at >95% confidence. If the edge energyis allowed to vary, then the best-fit value of 0.84± 0.04 keV isconsistent with an O viii edge.Heise et al. (1994) show that optically thick plasmas in thetemperature range 105 − 106 k are more efficient soft X-ray(0.1–1 keV) emitters than blackbodies, assuming plane parallelhydrostatic model atmospheres in which LTE determines thedegree of ionization. This conclusion has been extended to theNLTE case by Hartmann & Heise (1997) for both solar and LMCabundances. These models include free-bound opacity sourcesfor all relevant ions, but are still only first order approximationssince line blanketing has not been taken into account. In addi-tion, close to the Eddington limit the assumptions of hydrostaticequilibrium and plane parallel atmospheres are no longer valid.The above LTE and NLTE models were fit to the LECSCAL 87 spectrum. For the LTE case, we assume an LMC abun-dance of 0.25 times the solar value and a local gravity oflog(g) = 9, appropriate to WDs with mass ≥ 0.6M. Thefit results are however insensitive to abundance and adoptingTable 1. CAL 87 blackbody, LTE (Heise et al. 1994) and NLTE (Hart-mann & Heise 1997) WD model atmosphere spectral fit parametersBlackbody Blackbody with0.871 keV EdgeT (eV) 42±1311 59±2717NH (1022 cm−2) 1.00±0.050.11 0.53±0.580.02τ . . . >13R (cm) 2× 109 - 7× 1012 7× 107 - 6× 1010L (1036 erg s−1) 400 - 6× 108 4 - 1.5× 105χ2ν 1.18 0.93dof 14 13LTE NLTET (eV) 74.4± 1.7 57.3±1.92.4NH (1022 cm−2) 0.18±0.120.06 0.19±0.170.07R (cm) (9.1± 1.2)× 107 (1.67±0.350.25)× 108L (1036 erg s−1) 3.30± 0.54 3.89±0.890.69χ2ν 0.76 0.76dof 14 14solar abundance gives almost identical results. Models withlog(g) ≤ 8.5 cannot be made hot enough in hydrostatic equilib-rium (due to the Eddington limit) to fit the spectrum. We notethat models with log(g) >> 9 cannot be excluded since theycan fit the spectrum at higher effective temperatures and lowersource radii.Assuming a power-law spectrum with a photon index of 2.09(i.e. similar to that of the Crab Nebula) and a distance of 50 kpc,the 99% confidence upper-limit to any 2.0–10.0 keV emissionfrom CAL 87 obtained using MECS data is 1.3× 1034 erg s−1.3. DiscussionThe LECS spectrum of CAL 87 can be represented by all fourtypes of trial models and it is clear that a LECS spectrum withsignificantly greater exposure is required to meaningfully dis-criminate between these models based on fit quality alone. Thereare differences in the best-fit values of T determined using thedifferent models (see Table 1), with the blackbody fit giving thelowest T (and hence the largest source radius and luminosity)and the LTE fit the highest. The relatively large uncertaintiesin the best-fit parameters means that the luminosity and sizeof the X-ray emitting region are poorly constrained. The val-ues of χ2ν favor the interpretation of the spectrum in terms of amodel atmosphere fit with the LTE and NLTE fits both givingχ2ν values of 0.76. This should be compared with the blackbodyfit which gives a χ2ν of 1.18. Both WD model atmosphere fitsimply similar values of NH, while the temperature derived fromthe NLTE fit is significantly cooler (57.3±1.92.4 eV) than that de-rived assuming LTE (74.4 ± 1.7 eV). The best-fit blackbodyT derived here of 42±1311 eV is slightly higher, but consistentwith, those derived using the ROSAT PSPC of 31±1110 eV and34±810 eV (Kahabka et al. 1994).LETTERL36 A.N. Parmar et al.: A BeppoSAX observation of the super-soft source CAL 87The bolometric luminosity implied by the blackbody inter-pretation of >4 × 1038 erg s−1 is higher than the Eddingtonluminosity for a 1M object of 1.3× 1038 erg s−1. In addition,the required blackbody radius of >2 × 109 cm is significantlylarger than the expected WD radius (8.7× 108 cm for a 0.6MWD; Nauenberg 1972). In contrast, the fits using WD atmo-sphere models imply a lower luminosity, radius and temperatureof 2.7–4.8×1036 erg s−1, 8–20×107 cm and 55–76 eV, respec-tively. The WD mass can be constrained assuming that CAL 87is on the stability line (see Iben 1992, Fig. 2). The above tem-perature range corresponds to a WD of mass ∼1.2M whichhas a luminosity of 4–8×1037 erg s−1 while undergoing steadynuclear burning (see also Iben & Tutukov 1996). This is at least afactor 8 greater than the luminosity derived above. Since CAL 87has an orbital inclination of >70◦, it is possible that part of theemitting region is obscured, perhaps by the accretion disk. TheLECS spectrum of CAL 87 is therefore consistent with the as-sumption of a hot WD atmosphere heated by nuclear burning,but formally does not prove such an assumption.Acknowledgements. We thank the BeppoSAX Mission Director R.C.Butler and P. Giommi and F. Fiore for help with these observations.The referee, S. Rappaport, is thanked for helpful comments. PK is aHuman Capital and Mobility Fellow. The BeppoSAX satellite is a jointItalian and Dutch programme. IFCAI are supported by the Italian SpaceAgency (ASI) in the framework on the BeppoSAX mission.ReferencesAnders E., Grevesse N., 1989, Geochimica et Cosmochimica Acta 53,197Arnaud K.A., 1996, In: Jacoby G., Barnes J. (eds.) Astronomical DataAnalysis Software Systems V. ASP Conf. Series, 101, p17Boella G., Butler R.C., Perola G.C., et al., 1997a, A&AS 122, 299Boella G., Chiappetti L., Conti G., et al, 1997b, A&AS 122, 327Callanan P.J., Machin G., Naylor T., et al., 1989, MNRAS 241, 37pCowley A., Schmidtke P.C., Crampton D.A., et al., 1990, ApJ 350, 288Greiner J., Hasinger G., Kahabka P., 1991, A&A 246, L17Hartmann H.W., Heise J., 1997, A&A, in pressHeise J., van Teesling A., Kahabka P., 1994, A&A 288, L45Hellier C., Mason K.O., 1989, MNRAS 239, 715Iben I., 1982, ApJ 259, 244Iben I., Tutukov A.V., 1996, ApJS 105, 145Kahabka P., 1995, Supersoft X-ray sources in the LMC and SMC.In: Bianchini A., et al., (eds.) Cataclysmic Variables. Proc. of theAbano Terme Conf., Kluwar, p. 435Kahabka P., Trümper J., 1996, Supersoft ROSAT Sources in the Galax-ies. In: van Paradijs J., van den Heuvel E.P.J. Kuulkers E. (eds.) IAUSymp. 165, Compact Stars in Binaries. p. 425Kahabka P., Pietsch W., Hasinger G., 1994, A&A 288, 538Kylafis N.D., Xilouris E.M., 1993, A&A 278, L43Long K.S., Helfand D.J., Grabelsky D.A., 1981, ApJ 248, 925Morisson D., McCammon D., 1983, ApJ 270, 119Nauenberg M., 1972, ApJ 175, 417Parmar A.N., Martin D.D.E., Bavdaz M., et al., 1997, A&AS 122, 309Rappaport S.A., di Stefano R., Smith J.D., 1994, ApJ 426, 692Schandl S., Meyer-Hofmeister E., Meyer F., 1997, A&A 318, 73 472,p. 53Schmidtke P.C., McGrath T.K., Cowley A.P., Frattare L.M., 1993,PASP 105, 863Smale A.P., Corbet R.H.D., Charles P.A., et al., 1988, MNRAS 233,51Trümper J., 1983, Adv. Space Res. 2, 241van den Heuvel E.P.J., Bhattacharya D., Nomoto K., Rappaport S.A.,1992, A&A 262, 97White N.E., Giommi P., Heise J., Angelini L., Fantasia S., 1995, ApJ445, L125This article was processed by the author using Springer-Verlag LaTEXA&A style file L-AA version 3.LETTER

A BeppoSax observation of the supersoft source CAL87

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)A BeppoSax observation of the supersoft source CAL87Parmar, A.; Kahabka, P.; Hartmann, H.W.; Heise, J.; Martin, D.D.E.; Bavdaz, M.; Mineo, T.Publication date1997Published inAstronomy & AstrophysicsLink to publicationCitation for published version (APA):Parmar, A., Kahabka, P., Hartmann, H. W., Heise, J., Martin, D. D. E., Bavdaz, M., & Mineo,T. (1997). A BeppoSax observation of the supersoft source CAL87. Astronomy &Astrophysics, 323, L33-L36.General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:24 Jan 2024Astron. Astrophys. 323, L33–L36 (1997) ASTRONOMYANDASTROPHYSICSLetter to the EditorA BeppoSAX observation of the super-soft source CAL 87A.N. Parmar1, P. Kahabka2, H.W. Hartmann3, J. Heise3, D.D.E. Martin1, M. Bavdaz1, and T. Mineo41 Astrophysics Division, Space Science Department of ESA, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands2 Astronomical Institute and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403,1098 SJ Amsterdam, The Netherlands3 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands4 IFCAI/CNR, via U. Malfa 163, I-90146 Palermo, ItalyReceived 15 April 1997 / Accepted 6 May 1997Abstract. We report on a BeppoSAX Concentrator Spectrom-eter observation of the super-soft source (SSS) CAL 87. TheX-ray emission in SSS is believed to arise from nuclear burn-ing of accreted material on the surface of a white dwarf (WD).An absorbed blackbody spectral model gives a χ2ν of 1.18 anda temperature of 42±1311 eV. However, the derived luminosityand radius are greater than the Eddington limit and radius ofa WD. Including an O viii edge at 0.871 keV gives a signifi-cantly better fit (at >95% confidence) and results in more re-alistic values of the source luminosity and radius. We also fitWD atmosphere models to the CAL 87 spectrum. These alsogive reasonable bolometric luminosities and radii in the ranges2.7–4.8×1036 erg s−1 and 8–20×107 cm, respectively. Theseresults support the view that the X-ray emission from CAL 87results from nuclear burning in the atmosphere of a WD.Key words: X-rays: stars − accretion − binaries:close −stars:individual (CAL 87) − white dwarfs1. IntroductionThe Einstein observatory performed a survey of the Large Mag-ellanic Cloud (LMC) in which two sources with unusually softspectra, CAL 83 and CAL 87 were detected (Long et al. 1981).These sources emit little or no radiation at energies ∼>1 keVand became known as “super-soft” sources (SSS). SubsequentROSAT observations have revealed approximately 30 similarsources located in the Galaxy, the Magellanic Clouds, a globularcluster and M31 (see Kahabka 1995; Kahabka & Trümper 1996for recent reviews). SSS are hard to detect in the galactic planeSend offprint requests to: A.N. Parmar: aparmar@astro.estec .esa.nldue to the effects of interstellar absorption. Absorbed black-body spectral models give typical temperatures of ∼40 eV andbolometric luminosities of ∼1038 erg s−1.SSS were originally interpreted as due to scattering from anaccretion disk corona (e.g., Smale et al. 1988), or accreting neu-tron stars radiating near or above the Eddington limit (Greineret al. 1991; Kylafis & Xilouris 1993). Van den Heuvel et al.(1992) proposed that these are systems undergoing steady nu-clear burning of hydrogen accreted onto the surface of a whitedwarf (WD) with masses in the range 0.7–1.2 M. The masstransfer from a main-sequence or sub-giant companions is un-stable on a thermal time scale and for a narrow range of accretionrates, steady nuclear burning can take place. Evolutionary sce-narios for such systems are discussed in Rappaport et al. (1994).It is unlikely that SSS compose a homogeneous class and oneway of probing the nature of individual sources is by searchingfor the characteristic spectral signatures of nuclear burning on aWD. This burning takes place deep within the WD atmosphereat a large energy dependent optical depth. Photoelectric absorp-tion by highly ionized metals in the atmosphere can produceedges in the X-ray spectrum. These effects have been modeledassuming Local Thermodynamic Equilibrium (LTE) by Heiseet al. (1994) and more recently extended to the non-LTE (NLTE)case by Hartmann & Heise (1997).CAL 87 exhibits both X-ray and optical eclipses with a pe-riod of 10.6 hrs (Callanan et al. 1989; Cowley et al. 1990;Schmidkte et al. 1993; Kahabka et al. 1994), indicating an orbitalinclination of>70◦. The optical lightcurve shows a deep asym-metric primary minimum with a shallow, variable, secondaryminimum while the X-ray eclipse is narrower and shallow. Theoptical modulation may be due to obscuration by a structured ac-cretion disk (Schandl et al. 1997). Fitting an absorbed blackbodymodel to a 37 ksec exposure ROSAT Position Sensitive Pro-portional Counter (PSPC; 0.1–2.5 keV; Trümper 1983) CAL 87spectrum gives a best-fit temperature, T, of 35 eV and an equiv-LETTERL34 A.N. Parmar et al.: A BeppoSAX observation of the super-soft source CAL 87Fig. 1. Absorbed blackbody (left panel) and blackbody with a 0.871 keV absorption edge (right panel) model fits to the LECS CAL 87 spectrumFig. 2. Absorbed LTE (left panel) and NLTE (right panel) model fits to the LECS CAL 87 spectrumalent hydrogen column density, NH, of 1× 1022 H atoms cm−2(Hartmann & Heise 1997). CAL 87 has been a persistent X-raysource since its discovery in 1980.2. ObservationsThe scientific payload of the BeppoSAX X-ray Astronomy Satel-lite (Boella et al. 1997a) comprises four coaligned Narrow FieldInstruments, or NFI, including the Low Energy and MediumEnergy Concentrator Spectrometers (LECS and MECS). TheLECS is an imaging gas scintillation proportional counter sen-sitive in the energy range 0.1–10.0 keV with a circular fieldof view of 37′ diameter (Parmar et al. 1997). The backgroundcounting rate is 9.7 × 10−5 arcmin−2 s−1 in the energy range0.1–10.0 keV. The LECS energy resolution is a factor∼2.4 bet-ter than that of the ROSAT PSPC, while the effective area isbetween a factor ∼6 and 2 lower at 0.28 and 1.5 keV, respec-tively. CAL 87 was observed by the LECS as part of the ScienceVerification Phase between 1996 October 27 02:14 to October28 23:42 UTC. Due to the failure of a ground segment linkand an instrument anomaly, data between October 28 04:54 and10:55 UTC were lost. Good data were selected from intervalswhen the minimum elevation angle above the Earth’s limb was>4◦ and when the instrument’s settings were nominal usingthe SAXLEDAS 1.4.0 data analysis package. Since the LECSwas only operated during satellite night-time, this gave a totalon-source exposure of 39 ksec. The MECS is sensitive in theenergy range 1.5–10 keV, with energy and angular resolutionssimilar to the LECS (Boella et al. 1997b). The MECS observedLETTERA.N. Parmar et al.: A BeppoSAX observation of the super-soft source CAL 87 L35CAL 87 for a total exposure of 120 ksec, but did not detect thesource.Examination of the LECS image shows a source at a posi-tion consistent with (32′′ distant) that of CAL 87. A spectrumwas extracted centered on the source centroid using a radius of8′. This radius was chosen to include 95% of the 0.28 keV pho-tons. The spectrum was rebinned to have >20 counts in eachbin to allow the use of the χ2 statistic. The XSPEC 9.01 pack-age (Arnaud 1996) was used for spectral analysis together withthe response matrix from the 1996 December 31 release. Back-ground subtraction was performed using a standard blank fieldwith a 46 ksec exposure. The CAL 87 count rate above back-ground was 0.0076±0.0011 s−1. Examination of the extractedspectrum shows that the source was only detected in a narrowenergy range (see Figs. 1 and 2) and only the 17 rebinned chan-nels corresponding to energies between 0.2 and 1.5 keV wereused for spectral fitting.2.1. Spectral fitsIn order to compare the LECS spectrum with those obtainedfrom previous observations, we first fit an absorbed blackbodyspectral model to the data (Fig. 1). The photoelectric absorptioncoefficients of Morisson & McCammon (1983) together withthe solar abundances of Anders & Grevesse (1989) were used.An acceptable fit is obtained with χ2ν of 1.18 for 14 degrees offreedom (dof). The best-fit parameters are given in Table 1. Adistance of 50 kpc is assumed in order to derive the WD radius,R, and luminosity, L, and all uncertainties are quoted at 68%confidence. The spectrum shows evidence for an abrupt cutoff∼>0.8 keV and so an O viii edge with absorption depth τ at afixed energy of 0.871 keV, was added to the model. This edge isthe dominant spectral feature at energies∼>0.8 keV in many WDmodel atmosphere calculations (e.g. Heise et al. 1994; White etal. 1995). Including the edge gives a higher best-fit temperatureand improves the fit quality, resulting in a χ2ν of 0.93 (Table 1).The value of the F statistic of 4.76 indicates that the addition ofthe edge is significant at >95% confidence. If the edge energyis allowed to vary, then the best-fit value of 0.84± 0.04 keV isconsistent with an O viii edge.Heise et al. (1994) show that optically thick plasmas in thetemperature range 105 − 106 k are more efficient soft X-ray(0.1–1 keV) emitters than blackbodies, assuming plane parallelhydrostatic model atmospheres in which LTE determines thedegree of ionization. This conclusion has been extended to theNLTE case by Hartmann & Heise (1997) for both solar and LMCabundances. These models include free-bound opacity sourcesfor all relevant ions, but are still only first order approximationssince line blanketing has not been taken into account. In addi-tion, close to the Eddington limit the assumptions of hydrostaticequilibrium and plane parallel atmospheres are no longer valid.The above LTE and NLTE models were fit to the LECSCAL 87 spectrum. For the LTE case, we assume an LMC abun-dance of 0.25 times the solar value and a local gravity oflog(g) = 9, appropriate to WDs with mass ≥ 0.6M. Thefit results are however insensitive to abundance and adoptingTable 1. CAL 87 blackbody, LTE (Heise et al. 1994) and NLTE (Hart-mann & Heise 1997) WD model atmosphere spectral fit parametersBlackbody Blackbody with0.871 keV EdgeT (eV) 42±1311 59±2717NH (1022 cm−2) 1.00±0.050.11 0.53±0.580.02τ . . . >13R (cm) 2× 109 - 7× 1012 7× 107 - 6× 1010L (1036 erg s−1) 400 - 6× 108 4 - 1.5× 105χ2ν 1.18 0.93dof 14 13LTE NLTET (eV) 74.4± 1.7 57.3±1.92.4NH (1022 cm−2) 0.18±0.120.06 0.19±0.170.07R (cm) (9.1± 1.2)× 107 (1.67±0.350.25)× 108L (1036 erg s−1) 3.30± 0.54 3.89±0.890.69χ2ν 0.76 0.76dof 14 14solar abundance gives almost identical results. Models withlog(g) ≤ 8.5 cannot be made hot enough in hydrostatic equilib-rium (due to the Eddington limit) to fit the spectrum. We notethat models with log(g) >> 9 cannot be excluded since theycan fit the spectrum at higher effective temperatures and lowersource radii.Assuming a power-law spectrum with a photon index of 2.09(i.e. similar to that of the Crab Nebula) and a distance of 50 kpc,the 99% confidence upper-limit to any 2.0–10.0 keV emissionfrom CAL 87 obtained using MECS data is 1.3× 1034 erg s−1.3. DiscussionThe LECS spectrum of CAL 87 can be represented by all fourtypes of trial models and it is clear that a LECS spectrum withsignificantly greater exposure is required to meaningfully dis-criminate between these models based on fit quality alone. Thereare differences in the best-fit values of T determined using thedifferent models (see Table 1), with the blackbody fit giving thelowest T (and hence the largest source radius and luminosity)and the LTE fit the highest. The relatively large uncertaintiesin the best-fit parameters means that the luminosity and sizeof the X-ray emitting region are poorly constrained. The val-ues of χ2ν favor the interpretation of the spectrum in terms of amodel atmosphere fit with the LTE and NLTE fits both givingχ2ν values of 0.76. This should be compared with the blackbodyfit which gives a χ2ν of 1.18. Both WD model atmosphere fitsimply similar values of NH, while the temperature derived fromthe NLTE fit is significantly cooler (57.3±1.92.4 eV) than that de-rived assuming LTE (74.4 ± 1.7 eV). The best-fit blackbodyT derived here of 42±1311 eV is slightly higher, but consistentwith, those derived using the ROSAT PSPC of 31±1110 eV and34±810 eV (Kahabka et al. 1994).LETTERL36 A.N. Parmar et al.: A BeppoSAX observation of the super-soft source CAL 87The bolometric luminosity implied by the blackbody inter-pretation of >4 × 1038 erg s−1 is higher than the Eddingtonluminosity for a 1M object of 1.3× 1038 erg s−1. In addition,the required blackbody radius of >2 × 109 cm is significantlylarger than the expected WD radius (8.7× 108 cm for a 0.6MWD; Nauenberg 1972). In contrast, the fits using WD atmo-sphere models imply a lower luminosity, radius and temperatureof 2.7–4.8×1036 erg s−1, 8–20×107 cm and 55–76 eV, respec-tively. The WD mass can be constrained assuming that CAL 87is on the stability line (see Iben 1992, Fig. 2). The above tem-perature range corresponds to a WD of mass ∼1.2M whichhas a luminosity of 4–8×1037 erg s−1 while undergoing steadynuclear burning (see also Iben & Tutukov 1996). This is at least afactor 8 greater than the luminosity derived above. Since CAL 87has an orbital inclination of >70◦, it is possible that part of theemitting region is obscured, perhaps by the accretion disk. TheLECS spectrum of CAL 87 is therefore consistent with the as-sumption of a hot WD atmosphere heated by nuclear burning,but formally does not prove such an assumption.Acknowledgements. We thank the BeppoSAX Mission Director R.C.Butler and P. Giommi and F. Fiore for help with these observations.The referee, S. Rappaport, is thanked for helpful comments. PK is aHuman Capital and Mobility Fellow. The BeppoSAX satellite is a jointItalian and Dutch programme. IFCAI are supported by the Italian SpaceAgency (ASI) in the framework on the BeppoSAX mission.ReferencesAnders E., Grevesse N., 1989, Geochimica et Cosmochimica Acta 53,197Arnaud K.A., 1996, In: Jacoby G., Barnes J. (eds.) Astronomical DataAnalysis Software Systems V. ASP Conf. 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A BeppoSAX observation of the super-soft source CAL87

A BeppoSAX observation of the super-soft source CAL87

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